well this is rather vague...
you want to add a device, yes you could theoretically get something through usb that would otherwise be pci -- like an ethernet card.
you want to add memory, you'll need a new MB if you want to go over your current MB's limit.
no, MB's aren't by themselves expensive. the trouble is that upgrading a MB isn't generally worth it without updgrading other components as well... when you add up all the new parts you'll need, then it can get expensive.